1. Field of the Invention
The present invention relates generally to ink-jet printing and writing instrument servicing, and more specifically to a method for servicing ink-jet printhead nozzles.
2. Description of Related Art The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994) editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy [sic] Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988).
FIG. 1 (PRIOR ART) depicts an ink-jet hard copy apparatus, in this exemplary embodiment a computer peripheral printer, 101. A housing 103 encloses the electrical and mechanical operating mechanisms of the printer 101. Operation is administrated by an electronic controller 102 (usually a microprocessor or application specific integrated circuit ("ASIC") controlled printed circuit board) internally connected by appropriate cabling 104 and by input/output ports to a computer (not shown). It is well known to program and execute imaging, printing, print media handling, control functions and logic with firmware or software instructions for conventional or general purpose microprocessors or with ASIC's. Cut-sheet print media 105, loaded by the end-user onto an input tray 107, is fed by a suitable paper-path transport mechanism (not shown) to an internal printing station, or "print zone," where graphical images or alphanumeric text are created. A carriage 109, mounted on a slider 111, scans the print medium. An encoder 113, or other tracking device, is provided for keeping track of the position of the carriage 109 at any given time. A set 115 of one or more individual writing instruments 117A-117D, such as ink-jet pens or print cartridges, are releasable mounted in the carriage 109 for easy access (generally, in a full color system, inks--or other equivalent colorant, toner, or the like--for the subtractive primary colors, cyan, yellow, magenta (CYM) and true black (K) are provided). Once a printed page is completed, the print medium is ejected onto an output tray 119. In the state of the art, a printer 101 can have a variety of print modes related to the quality of the output desired by the end-user or to a specific print medium in use, e.g., plain paper, transparencies, photographic paper, and the like. Printing is accomplished generally by scanning and firing ink droplets; a unidirectional scan across the page is referred to as a sweep; the height of the writing instrument, e.g, from less than an inch to a full-page high nozzle array, determines the printed swath height of a given sweep.
Print cartridges are generally fully self-contained inking units intended for one-time use and replacement. Ink-jet pens are inking units which separate semi-permanent printhead mechanisms from the ink supply either by having an ink reservoir (not shown) off-axis from the pen coupled thereto by appropriate fluidic linkage, or a separate, snap-on or press-fit, ink supply for each pen. Pens tend to be constructed to use free-ink in a contained but unencumbered liquid form rather than in a saturated material such as polyurethane foam used in some print cartridges to facilitate the repeated ink supply replacements. In both cartridges and pens, their printheads generally require a mechanism to prevent the free flow of ink through the arrays of nozzle orifices thereon when the printhead is not activated. Without such control, ink may leak, or "drool," onto the printing surface or into the printer mechanism. Such leaking ink may also build up and cake onto the printhead itself, impairing proper operation.
Complex pen service stations are often provided as part of the hard copy apparatus where printheads are capped to prevent drooling and caking when not printing. The service station devices are generally located off to one side of the print zone of the apparatus (see e.g., FIG. 1, arrow 121). The printhead nozzles also can be wiped or activated to "spit" away excess ink into a spittoon and clear the nozzles while in the service station. [Service stations and their multifunctional operations are generally described in the literature and patents, such as the common assignee's U.S. Pat. No. 4,853,717, incorporated herein by reference.] Most printers have a regularly timed spit that occurs at the same interval regardless of the printing operation in progress. Generally, a simple countdown timer is started when a pen is decapped; when the timer has run-out, an interrupt signal is sent to the printing controls and the pen is returned to the service station to spit all nozzles.
A common modification of the fixed spit interval is to signal for a service interrupt when the printhead is not otherwise engaged, e.g., while the printer is loading a next sheet of print medium. When printing of the new sheet is initiated, the fixed spit timer is reset.
In some print modes, e.g., a DRAFT mode, throughput is more important than quality. In other print modes, e.g., a HIGH DEFINITION PHOTO mode, print quality is paramount with less emphasis on total page printing time for the current page. Such print quality modalities can be taken into account for varying the time between service spitting the nozzles, e.g., lengthening the interval in a DRAFT mode. The commonly used timed spit interval can be modified even within a print mode to improve throughput and to avoid print defects. For example, the timer may signal for a spit while the printhead is printing on the side of a page distal from the service station; the control logic permits the print of the sweep which repositions the printhead until the carriage has returned the side of a page proximate the service station. Moreover, some print modes, such as multi-pass print swaths, are especially sensitive to the ink drop firing order and timing. To avoid service pause related defects, the control logic may over-ride a service time-out until a printing pause opportunity occurs; e.g., on a multiple photographic image page, between the end of one image and the start of the next. However, depending on the ink formulations, the simple countdown timer methodology does not address variations in decap performance between inks, e.g., there may be a disparity between the length of time yellow ink may spend decapped without printing and the length of time for cyan ink. The problem is even more egregious in documents such as business graphics which tend to have both regions of text and color graphics or ink-jet printed photographs on the same page.
Another problem is known as wait banding, which occurs when pauses in a printing routine results in a sweep where wet colorant and dry colorant overlap. This then appears as a print artifact of either a value or hue shift in a single band across the page.
Thus, there is a need for a method of service spitting for ink-jet printheads which addresses the needs of each ink formulation and which addresses individual needs of each print mode available in the specific printer.